1. Field of the Invention
The present invention related to an LED head which is assembled into an electrophotographic printer and includes an optical lens assembly that focuses light emitted from LEDs of the LED head on the surface of a photoconductive drum.
2. Description of Conventional Art
With a conventional electrophotographic printer, an LED head incorporates a lens holder which receives an optical lens assembly therein and holds it in position with respect to the LEDs to accurately focus the images of the LEDs on the surface of a photoconductive drum. Accurately focusing the images of the LEDs on corresponding points of the photoconductive drum is one of the major factors that determines the print quality of an electrophotographic printer.
FIG. 17 is a cross sectional view of the conventional LED head. The LED circuit board 7 has a row of LED arrays 6 mounted thereon and longitudinally extends in a direction perpendicular to the page of FIG. 17. There is a lateral small gap between the edge of the LED circuit board 7 and the SLA holder 3. This gap is due to manufacturing errors. The SLA (selfoc lens array) 4 extends in a direction parallel to the row of the LED arrays 6. The SLA 4 is supported between two opposing walls 3f and 3g of an SLA holder 3 in a sandwiched relation, the two walls 3f and 3g extending in directions parallel to the row of the LED arrays 6. The wall 3f has a plurality of resilient members 3d, for example, three resilient members, that resiliently engage one side of the SLA 4 to urge the SLA 4 against the wall surface 3e of the other wall 3g. The SLA holder has projections 3a outwardly extending from the opposing walls 3f and 3g, which engage cutouts 2a (FIG. 4) in a base 2 when assembled to the base 2. The walls 3g and 3f has abutting surfaces 3c, respectively, which are in pressure contact with the top surface of the LED circuit 7 to firmly hold the LED circuit board 7 when the SLA holder 3 has been assembled into the a base 2, thereby accurately positioning the SLA 4 relative to the LED arrays 6.
FIG. 18 is a top view of the SLA 4 and SLA holder 3 when SLA 4 is urged by three resilient members 3d of the SLA holder 3.
When the SLA 4 is inserted between the two walls 3f and 3g of the SLA holder 3, the SLA holder 3 is deformed by forces F1, F2, and F3 applied by the three resilient members 3d, the two walls being pushed away from each other by the SLA 4 to extend in directions shown by arrows E and F. The deformation of the two walls in the directions shown by arrows E and F causes the SLA holder to deform at its lower end in a direction shown by an arrow G (FIG. 17). As a result, the opposing walls 3f and 3g are inclined. Such an inclination of the opposing walls 3f and 3g causes the SLA 4 to significantly deform as shown in FIG. 18. The deformation of the opposing walls 3f and 3g gives rise to the problem that the images of LED arrays 6 are not properly focused at corresponding points on the surface of the photoconductive drum 8.
The SLA holder 3 is fixed to the base 2 at longitudinal ends thereof and therefore the deformation of the SLA 4 near the longitudinal ends of the SLA holder 3 is not significant. Thus, the images of the LED arrays 6 near the longitudinal ends are formed on or very close to a point 8a of the photoconductive drum 8, the light being substantially normal to the surface of the photoconductive drum 8.
The SLA holder 3 is noticeably deformed by forces F1, F2, and F3 applied by the spring members 3d, so that the space 1a becomes wider toward a longitudinal middle thereof. The deformation of the SLA holder 3 causes the SLA 4 to tilt with respect to the LED arrays 6 when assembled into the LED head.
FIG. 19 shows the SLA 4 when it is not correctly positioned but inclined with respect to the LED arrays 6.
If the SLA 4 is correctly positioned, then optical axis of the SLA 4 is normal to the point 8a so that the image of the LED array 6 is formed at the point 8a. If the SLA 4 is inclined, then the optical axis of the SLA 4 aims at a point 8b where the SLA 4 focuses an image of a point 9 near the LED array 6 at a point 8c on the photoconductive drum 8, and the image of the LED array 6 is formed at the point 8b. It is to be noted that the images at 8c and 8b are behind the focal plane of the SLA 4 and is circumferentially away from the point 8a.
Thus, when the SLA 4 is correctly positioned, the images of all the LEDs on the LED arrays 6 are focused on the surface of the photoconductive drum 8 to form a line of points parallel to the axis of rotation of the drum 8. If the SLA 4 is inclined, then the line of images formed on the photoconductive drum 8 is curved so that the focused images are increasingly away from a line parallel to the axis of rotation of the photoconductive drum 8 toward the longitudinal middle of the photoconductive drum 8. Such deformation and/or inclination of the SLA 4 causes distorted images and is detrimental particularly in a tandem type color printer where registration of images of respective colors is of great importance. Misregistration of color images due to the deformation and/or inclination of the SLA 4 results in poor print quality.